This invention relates to a numerical path control system for a machinetool, particularly for a crankshaft-milling machine, which control system comprises at least two pulse-controlled drive means for moving a tool along respective coordinate axes; and arithmetic unit, which in dependence on tool data and workpiece data provides section end point data indicating the ends of sections of a path along which a tool is to be moved, and change point data, which indicate change point positions at which the speed and/or the direction of movement of the tool is to be changed, which section end point data and change point position data are related to said coordinate axes; and interpolators, which in dependence on said input data and the computed section end point data control the drive means to move the tool along a path deliver control pulses at a clock frequency which has been selected for the path section along which the tool is being moved and controls the driving speed.
When it is desired, e.g., to use an internal milling cutter to machine the crankpin of a crankshaft, it is necessary first to move the milling tool into engagement with the crankpin and then to move the milling tool around the crankpin so that the tool can recess the crankpin along a straight path which is inclined at the position angle of the crankpin. This path is succeeded by a circular path for the actual machining. In such case the tool path depends not only on the dimensions of the tool but also on the desired dimensions of the workpiece so that the actual course of the tool path can be determined for each workpiece of a given kind by a certain sequence of arithmetic operations, in which dimensions of the tool and workpiece data taken from a drawing have been taken into account. This fact is utilized in known control systems for shaft-milling machines in that a suitable computer is provided with intermediate storage means. Because the tool is usually moved along its path at different speeds during recessing and during the subsequent machining, such computer can be used to determine the positions at which the speed and direction of movement are changed if it is fed with input data representing the extents of movement in the several speed stages, for recessing, for creeping. In that case, the drive means for moving the tool along respective coordinate axes can be controlled in dependence on the computed path position data, which are related to the coordinate axes. For this purpose, separate interpolators are provided for each path section having a given configuration, and these interpolators deliver suitable control pulses to the drive means. The configuration of the path section will then depend on the distribution of the drive control pulses between the coordinate axes and the speed of the tool will depend on the frequency of the control pulses. The interpolators can control the drive means only to move the tool along a path having a predetermined confoguration, e.g., an arc of a circle or a straight line, and the extent of the movement along the path is determined by the feed data or by the data which have been determined by the computer. These interpolators are relatively expensive and must be provided with a clock system for the several interpolator sections so that the control pulses can be delivered in steps to the drive means for the movement along one coordinate axis or the other when the deviation from the desired value has reached the limit which calls for a change over to the movement along the other coordinate axis. Such clock systems can easily be deranged and the actual value must also be compared for a check of the tool movement.
The known control system are expensive and liable to be deranged and the provision of separate interpolators for every path configuration results in a discontinuity at the point where a change is effected from one path configuration to another. That discontinuity reduces the precision of the machining operation. Besides, the known control systems cannot correct defects of the machine tool, e.g., the backlash which occurs during the change of direction of a coordinate drive means during the interpolation of a circle.